1. Field of the Invention
The present invention relates to a sintered alloy which is suitable for a turbo component for turbocharger, particularly a heat resistant bearing and the like which require heat resistance, corrosion resistance and wear resistance, and a method for manufacturing the sintered alloy.
2. Background of the Invention
Generally, in a turbocharger provided in an internal combustion engine, a turbine is rotatably supported by a turbine housing connected with an exhaust manifold of the internal combustion engine. An exhaust gas flowed in the turbine housing is flowed in the turbine from the outside thereof and emitted in the axial direction thereof while the turbine is rotated. Then, air to be supplied into the internal combustion engine is compressed by the rotation of an air compressor which is provided at the same shaft in the opposite side of the turbine. In such a turbocharger as described above, when the exhaust gas is flowed in the turbine housing from the exhaust manifold, the exhaust gas is separated by the nozzle vanes and valves to control the inflow therein in order to obtain the stable boost pressure and prevent the damages of the turbocharger and the engine.
The bearings supporting the corresponding valves are subject to the exposure of high temperature exhaust gas and requires excellent wear resistance. Moreover, since the bearings are exposed to the air with the turbine housing and thus located under corrosion atmosphere causing salt damage, the bearings requires excellent corrosion resistance.
Moreover, since the turbo component for turbo charger is contacted with the exhaust gas as the high temperature corrosion gas, the turbo component requires heat resistance and corrosion resistance and wear resistance because the turbo component is slid relative to nozzle vanes and valve shafts. In this point of view, conventionally, high chrome cast steel, wear-resistant material made of JIS (Japanese Industrial Standards) SCH22 to which chrome surface treatment is conducted for the enhancement of corrosion resistance and the like are used. Moreover, as an inexpensive wear-resistant component having heat resistance, corrosion resistance and wear resistance is proposed a wear-resistant sintered component in which carbide is dispersed in the base material of a ferric steel material (Refer to Patent document No. 1).
However, since the sintered component disclosed in Patent document No. 1 is formed through liquid phase-sintering, the sintered component may be machined as the case of severe dimensional accuracy. Since the large amount of hard carbide is precipitated in the sintered component, the machinability of the sintered component is not good and thus required to be improved. Moreover, the turbo component is normally made of austenitic heat-resistant material, but the turbo component disclosed in Patent document No. 1 is made of ferritic stainless material. In this case, since the thermal expansion coefficient of the turbo component is different from those of the adjacent components, some spaces are formed between the turbo component and the adjacent components, causing the insufficient connections between the turbo component and the adjacent components and rendering component design available in the turbocharger difficult. It is therefore desired that the turbo component has a similar thermal expansion coefficient to those of the adjacent components made of austenitic heat-resistant material.
On the other hand, since a transportation machine such as a vehicle where the turbo charger is mounted is used within a wide range environmental from warm area to cold area, the turbo charger also is required to have excellent wear resistance and corrosion resistance within the wide range environmental. For example, in a cold district, sodium chloride (NaCl) or calcium chloride (CaCl2) is scattered as antifreeze or liquefacient on the road surface. Since a large amount of salt water with high concentration exists on the road surface by the melting of the snow and ice, the high concentration salt water is splashed and adhered to the rear side of the transportation machine when the transportation machine runs on the road surface. A large amount of chloride ion contained in the high concentration salt water breaks the passive film formed at the surface of stainless steel and causes the corrosion against the stainless steel. Therefore, when the heat resistant bearings for turbo charger are made of the stainless steel, some corrosion problems such as salt damage are caused.
It is said that the salt damage corrosion mechanism is originated from that the passive film (Cr2O3) is reacted with H2O in addition to Na of NaCl to form water soluble Na2CrO4 to be able to melt the passive film. Then, it is considered that Cr is appropriately supplied from the interior of the stainless steel to the passive film with the melting of the passive film so that the amount of Cr in the stainless steel becomes short.
Under the circumference of salt damage corrosion, corrosion is caused even in the sintered alloy disclosed in Patent document No. 1, so that such a new sintered alloy as having wear resistance and corrosion resistance.    Patent document No. 1: Japanese Patent publication No. 3784003